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Appliance Transformers

  1. May 14, 2006 #1


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    In regards to the small, black box transformer-rectifiers one sees all the time on things like printers, speakers, battery chargers etc... for computers and other appliances, how tight are they on specifications for output? For example, if I have a device that is rated 12V DC and 4 amps, how sensitive are the appliances in reference to that rating? Do they have a standard spec for things like ripple?

    The reason I am asking is that 1) I have always been curious about this and 2) I have come across a need to find a replacement that I can not get from the OEM.

    So my device states the rating of 12VDC and 4 amps. If I were to track down a similar unit at Radio Shack, do I stand a chance of damaging the unit?

    If anyone can give a nice little primer on these transformer rectifiers, I would be much appreciative.
  2. jcsd
  3. May 14, 2006 #2
    Your answer actually depends on the voltage/current sensitivity tolerances of the connected devices' input-power electronic circuitry.
    Some connected devices are electronically "forgiving", others have very minimal "wiggle-room" outside specifications for input.
  4. May 14, 2006 #3


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    Hi Fred,

    50W is a lot to ask from a wall wart, but switching versions will support that power level without the big bulk. I'd feel comfortable with the Radio Shack equivalent -- worst case is undervoltage, not overvoltage.
  5. May 14, 2006 #4


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    There are two basic types of these things.
    The older transformer design (heavy and big at 4 amps)
    and the newer switching types.

    If your unit is a single voltage output transformer design then voltage regulation and filtering are non existent.
    I've never seen one otherwise, except for some multi voltage output computer packs.
    Basically its just a fuse, 12v transformer and a fullwave rectifier.
    The measured open circuit voltage will be around 18v.
    Anything that matches the stated voltage with >= current output will work.
    Note: some are AC output -- no rectifier, so watch out for that.

    If its the transformer type and you are worried, crack it open and check.
    Chances are it just popped the fuse anyway so if you're careful opening it you could just replace the fuse.
    The smaller current transformer devices often have the fuse built into the transformer.

    The small light switching units are highly regulated and filtered.
    Just get one where the voltage matches. Ouput current can be higher.

    You should be able to replace the old transformer type with
    one of these, but I wouldn't go the other way.

    Oh yea. Watch out for the connector polarity.
  6. May 15, 2006 #5


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    Thanks! That is something I was wondering if it would happen.

    Thanks guys. I appreciate the help. I really hate to see a, literal, black box and not know anything about it.

    Just to fill you in, I actually have two applications that I am looking for replacements:

    1) My cordless drill's battery charger is always going south on me. I am on #3 and it looks like that one has gone bad as well. I have read that the Craftsman battery chargers are pretty much crap, so I want to get a different brand and try it.

    2) The 12VDC, 4amp unit is, believe it or not, for a flat panel, LCD computer monitor my wife found for almost nothing. It's brand new, just no power supply. I was shocked to see this thing require this kind of power input.
  7. May 15, 2006 #6


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    "I was shocked to see this thing require this kind of power input."

    Well, lots of companies don't want to pay the big bucks to get their power supplies UL approved, so they purchase OEM supplies that are already UL approved.

  8. May 16, 2006 #7


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    The majority of that power goes to the backlight.
    The TFT works as a programable filter in front of the lightsource.

    For this application you could jack into a spare drive connector in the PC.
    One of the lines (Yellow IIRC) is 12v, Black is ground.
    Shouldn't be any problem with an additional 50w.
  9. May 16, 2006 #8


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    Now that is a darned fine idea. Excellent. I think I'm going to take a look into doing that. I have an extra drive power cable bundle in there as we speak. Thanks again!
  10. May 16, 2006 #9


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    After replacing any number of power supplies over the years (I was the network administrator for a medical practice), I can tell you that I personally would not strain a consumer-grade PC's power supply with an additional 50-watt load. That's asking for trouble.
  11. May 16, 2006 #10


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    Don't know how much a strain that is.
    Unless it's a real old PS (they maxed out around 5A@12v) newer ones seem to be good for 12 to 15 amps @ 12v.

    Read the label on the PS.

    With a HD + DVD + 4A it comes to about 6A or 50% of max.

    My son used to do some of that, said that certain manufactures were junk.
    It was always the same ones that died.
    Since I actually had one of the "bad" PS - Loading didn't seem to be an issue.
    The cheap caps just dried out eventually.
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